Nanostructured CeO2 with different morphologies for methoxycarbonylation of 2,4-toluene diamine with dimethyl carbonate

The methoxycarbonylation of 2,4-toluene diamine (TDA) to dimethyl toluene-2,4-dicarbamate (TDC) is an important reaction step in the phosgene-free synthesis of toluene diisocyanate (TDI). To further clarify the catalytic mechanism and enhance the catalytic performance of CeO2, a series of CeO2 catalysts with different morphologies were prepared and their catalytic performance in TDA methoxycarbonylation was evaluated. The results shows that spherical cerium dioxide (CeO2-S) has better catalytic performance. Combined with characterization methods such as TEM, XRD, BET, TPD, XPS, Raman spectra and H2-TPR, the reasons for the differences in catalytic performance of different CeO2 were analyzed, and it is found that the amount of medium-strong acid sites, lattice oxygen content and specific surface area of CeO2 are positively correlated with their catalytic performance. Under optimized conditions, TDA was completely converted, the yield of TDC reached 86.8%, and the total yield of methyl carbamates (including monomethyl carbamate and TDC) reached as high as 96.5%. CeO2-S could be reused four times without significant change in activity. Through designed experiments, it is determined that both pathways with methyl 3-amino-4-methyl-N-phenyl carbamate (4-TMC) and methyl 2-methyl-5-amino-N-phenyl-carbamate (2-TMC) as intermediates could eventually generate TDC. Finally, a synergistic catalytic mechanism of acid site and lattice oxygen in CeO2 for the methoxycarbonylation of TDA was established.